@media (min-width: 1030px) { 
.header {
background-image: url(https://www.sport-xxl-mode.de/images/header-neu/header-img/pc-tennis.gif);
}
}
@media (max-width: 600px) { 
.header {
background-image: url(https://www.sport-xxl-mode.de/images/header-neu/header-img/smart-tennis.gif);
}
}